A survey of the health status of parents and grandparents of cystathionine synthase deficient patients has been initiated. The aim is to determine whether heterozygotes for this genetic defect are at increased risk for coronary artery disease. By appropriate physican contact we have, or soon will have, disbursed questionnaires to 200-250 such families. Depending upon the incidence of coronary disease in the control population, this survey should provide at least 80 percent chance of detecting a statistically significant (p less than .05) three-fold increase in the incidence of coronary artery disease among parents and/or grandparents. Work is now underway to obtain comparable data for three control groups: relatives of patients with (a) cystinosis, (b) phenylketonuria, and (c) achondroplastic dwarfism. The latter is the group of choice for several reasons and the major effort initially will focus upon this group.